1. Field of the Invention
This invention relates to an aqueous epoxy resin composition, more specifically it relates to an aqueous resin composition which is excellent in stability of an aqueous dispersion and processability, curability and corrosion resistance of a coated film, and is suitable for a metal can, particularly for inner surface coating of food cans and beverage cans excellent in flavor maintaining property.
2. Background Information
This invention also relates to a method for forming a coating film on an inner surface of a can body such as two-piece (2P) can used as cans for beverage or food, more specifically it relates to a method for forming a coating film of a can body excellent in adhesiveness and corrosion resistance, which is applicable with coatings of a pre-coat using an aqueous coating composition comprising an acryl-epoxy-acryl resin and a top coating layer.
A coating for a metallic can is required to be sufficiently durable to various conditions during working and commercial distribution of the metallic can and a content therein. It is also important to deal with various problems such as hygiene of working conditions and prevention of fire and explosion, and particularly important for beverage or food cans to satisfy the requirements on food hygiene. Thus, in recent years, an aqueous coating has widely been used.
In such a situation, an aqueous epoxy resin coating composition has been particularly attracted to attention due to its easiness in handling and various compositions have been proposed. For example, in Japanese Provisional Patent Publication No. 135260/1984 (which corresponds to U.S. Pat. No. 4,446,258 of Chu et al.), there is disclosed an aqueous resin composition in which an acrylic resin-modified epoxy resin having a high acid value and an acrylic resin containing no acidic group are dispersed in an amine aqueous solution.
However, these prior art aqueous epoxy resin compositions have problems that they are generally insufficient in dispersion stability, poor in processability or corrosion resistance, or low in flavor maintaining property.
In the aqueous dispersion coating composition of the prior art, when an acrylic resin having a low glass transition temperature is used in order to improve processability of a coated film, stability of aqueous dispersion lowered so that processability and stability of aqueous dispersion are imcompatible with each other.
On the other hand, a two-piece can has generally been used more widely than the conventional three-piece (3P) can since the former is inexpensive, high quality and beautiful. However, it has been produced by Draw & Ironing (hereinafter abbreviated to as "DI") processing using a tin plate or an aluminum plate as a raw material so that some problems, which should be solved, remain in coating of such a DI can.
Thus, In Japanese Patent Publication No. 4753/1985, there is disclosed a process for producing a highly corrosion resistant DI can comprising coating a paint for cans, containing at least one of an epoxy group, a hydroxyl group and a carboxyl group on a steel plate containing a metallic chromium layer and a hydrated chromium oxide layer, then subjecting to DI processing, coating once again and baking. Also, in Japanese Patent Publication No. 25784/1990, there is disclosed a coated steel plate for DI cans improved in adhesiveness and corrosion resistance, in which a steel plate subjected to specific plating and conversion treatment is coated with a thermoplastic polyester which is capable of orientating. Further, in Japanese Provisional Patent Publication No. 92850/1986, there is disclosed a coated steel plate for DI cans having excellent corrosion resistance, in which a steel plate subjected to specific conversion treatment is coated with a vinyl chloride series organo sol coating containing a resol type phenol resin.
However, each of the above prior art uses an organic solvent type coating so that they are not preferred in the point of fire, working environment and global environment. Further, they are not necessarily sufficient in interlaminar adhesiveness or high temperature resistance for sterilization.